The present invention relates generally to the field of satellite positioning systems (SPS), such as global positioning system (GPS) receivers, and more particularly to processing of SPS signals.
Global Positioning System (GPS) receivers normally determine their position by computing times of arrival of signals transmitted simultaneously from a multiplicity of GPS (or NAVSTAR) satellites. These satellites transmit, as part of their message, both satellite positioning data as well as data on clock timing, so-called xe2x80x9cephemerisxe2x80x9d data. The process of searching for and acquiring GPS signals, reading the ephemeris data for a multiplicity of satellites and computing the location of the receiver from this data is time consuming, often requiring several minutes. In many cases, this lengthy processing time is unacceptable and, furthermore, greatly limits battery life in miniaturized portable applications.
GPS receiving systems have two principal functions. The first is the computation of the pseudoranges to the various GPS satellites, and the second is the computation of the position of the receiver using these pseudoranges and satellite timing and ephemeris data. The pseudoranges are simply the times of arrival of satellite signals measured by a local clock. This definition of pseudorange is sometimes also called code phase. The satellite ephemeris and timing data is extracted from the GPS signal once it is acquired and tracked. As stated above, collecting this information normally takes a relatively long time (30 seconds to several minutes) and must be accomplished with a good received signal level in order to achieve low error rates.
Most GPS receivers utilize correlation methods to compute pseudoranges. These correlation methods are performed in real time, often with hardware correlators. GPS signals contain high rate repetitive signals called pseudorandom (PN) sequences. The codes available for civilian applications are called C/A (coarse/acquisition) codes, and have a binary phase-reversal rate, or xe2x80x9cchippingxe2x80x9d rate, of 1.023 MHz and a repetition period of 1023 chips for a code period of 1 millisecond. The code sequences belong to a family known as Gold codes, and each GPS satellite broadcasts a signal with a unique Gold code.
For a signal received from a given GPS satellite, following a downconversion process to baseband, a correlation receiver multiplies the received signal by a stored replica of the appropriate Gold code contained within its local memory, and then integrates, or low-pass filters, the product in order to obtain an indication of the presence of the signal. This process is termed a xe2x80x9ccorrelationxe2x80x9d operation. By sequentially adjusting the relative timing of this stored replica relative to the received signal, and observing the correlation output, the receiver can determine the time delay between the received signal and a local clock. The initial determination of the presence of such an output is termed xe2x80x9cacquisition.xe2x80x9d Once acquisition occurs, the process enters the xe2x80x9ctrackingxe2x80x9d phase in which the timing of the local reference is adjusted in small amounts in order to maintain a high correlation output. The correlation output during the tracking phase may be viewed as the GPS signal with the pseudorandom code removed, or, in common terminology, xe2x80x9cdespread.xe2x80x9d This signal is narrow band, with a bandwidth commensurate with a 50 bit per second binary phase shift keyed (BPSK) data signal which is superimposed on the GPS waveform.
The correlation acquisition process is very time consuming, especially if received signals are weak. To improve acquisition time, most GPS receivers utilize a multiplicity of correlators (up to 36 typically) which allows a parallel search for correlation peaks.
Conventional GPS receiving equipment is typically designed to receive GPS signals in open spaces since the satellite signals are line-of-sight and can thus be blocked by metal and other materials. Improved GPS receivers provide signal sensitivity that allows tracking GPS satellite signals indoors, or in the presence of weak multipath signals or signals that are pure reflections. The ability to acquire such weak GPS signals, however, typically causes other problems. For example, the simultaneous tracking of strong and weak signals may cause the receiver to lock on to a cross-correlated signal, which is not a true signal. Instead of finding a weak true peak, a stronger cross-correlated peak may be acquired. Tracking a weak satellite signal does not guarantee that it is a direct signal. This weak signal may be a reflected signal or a combination of direct and indirect signals. The combined signals are referred to as multipath signals. The path of the reflected signal is typically longer than the path of the direct signal. This difference in path length causes the time-of-arrival measurement of the reflected signal to be delayed or the corresponding code phase measurement to contain a positive bias. In general, the magnitude of the bias is proportional to the relative delay between the reflected and direct paths. The possible absence of a direct signal component makes the existing multipath mitigation techniques (such as a narrow correlator or a strobe correlator) obsolete.
It is therefore desirable to provide a measurement processing algorithm that optimally utilizes various types of available data to achieve an optimal location solution.
A method and apparatus for measurement processing of SPS signals is disclosed. In one embodiment of the present invention, a plurality of GPS signals from a corresponding plurality of GPS satellites are received in a GPS receiver. The signal environment corresponding to the location in which the GPS receiver is located is characterized to produce signal environment data. In one exemplary embodiment, an information source, such as a cellular network based database, is searched to retrieve the signal environment data given an approximate location of the GPS receiver. This approximate location may be specified by a location of a cell site which is in cellular radio communication with a cellular communication device which is co-located with the GPS receiver. One or more parameters related to signal characteristics of the satellite signals are defined. Threshold values for the parameters are determined using the signal environment data. Code phases corresponding to times of arrival of respective satellite signals from the plurality of satellites are measured. Data representing measured times of arrival are tested using threshold values for the parameters to produce a set of times of arrival from which a location solution for the GPS receiver is calculated.
In another embodiment of the invention, the signal environment corresponding to the location in which a GPS receiver is located is characterized to produce signal environment data. This signal environment data reflects the manner in which SPS signals are propagated in the location. The signal environment data is used to determine at least one processing value which is, in turn, used to process data representative of SPS signals received by the GPS receiver.
In one particular embodiment of the invention, a cell based information source (e.g. a cellular phone network based database) is used to determine a signal environment data which represents a manner in which SPS signals are propagated in a location at which an SPS receiver is located, and SPS signals by the SPS receiver at this location are processed in a manner specified by the signal environment data.
In another embodiment of the invention, a method of processing SPS signals determines the existence of two (or more) correlation peaks from the same set of SPS signals from a first SPS satellite. A measurement representing a time-of-arrival of the set of SPS signals is derived from the two (or more) correlation peaks; typically the earlier correlation peak represents a direct path of the set of SPS signals, rather than a reflected path, and the earlier correlation peak is used to derive the measurement representing the time-of-arrival of the set of SPS signals.
Other features and embodiments of the present invention will be apparent from the accompanying drawings and from the detailed description which follows.